1. Field of the Invention
The present invention relates to a process for manufacturing an aluminum alloy material for forming which has excellent formability in press working, shape fixability and bake hardenability, and which is especially suitable for the manufacture of transport machinery, such as the body sheet material of automobiles.
2. Description of the Prior Art
Various types of aluminum alloys have heretofore been developed and used as the material of transport machinery, such as the body sheet material of automobiles. Especially, in recent years, a tendency toward using aluminum alloys instead of steel materials to obtain a light-weight structure with respect to various parts is very conspicuous in compliance with the tightening of legal regulations established as the countermeasures against earth warming.
For example, the body sheet materials of automobiles should satisfy the requirements for (1) formability, (2) shape fixability (accurate reproduction of the shape of press dies in press working), (3) high strength, (4) dentability, and (5) corrosion resistance, etc.
Under these circumstances, in Japan where the requirements from the press work industry are strict, the development of the body sheet materials of automobiles or the like has mainly been directed to 5000 series Al-Mg-Zn-Cu alloys (see Japanese Patent Application Laid-Open Nos. 53-103914 and 58-171547) and Al-Mg-Cu alloys (see Japanese Patent Application Laid-Open No. 1-219139) having excellent formability, and these body sheet materials have been mass-produced and put to practical use.
By contrast, in the United Stated and Europe, ( 6009, 6111 and 6016 alloys have been developed as the 6000 series Al-Mg-Si alloys having high strength. These alloys acquire high strength by heat treatment at 200.degree. C. for about 30 minutes in the baking step (bake hardening). The increase in the strength enables a marked decrease in thickness from 5000 series alloys, i.e., a light-weight structure, to be attained. However, in Japan, since the bake temperature is as low as about 170 to 180.degree. C., it is unexpectable to achieve a satisfactory high strength by 30-minute heating with the current 6000 series alloys or the current manufacturing process. Moreover, the current 6000 series alloys suffer from room temperature age hardening, though slightly, and have problems that the formability is poor and the corrosion resistance is also relatively poor. Therefore, in Japan where the requirements for various performances are strict, the 6000 series alloys have no significant advantage over the 5000 series alloys so far as the baking step is conducted at a higher temperature or for a longer period of time as compared with the prior art, so that the former has been hardly employed.
On the other hand, the shape fixability can be improved as the Young's modulus is increased and the yield strength is decreased (see SAE Paper No. 890719). Because the Young's modulus of an aluminum alloy is 70000 MPa which is about one third of 210000 MPa for steel, it is impossible to obtain a material having the same shape fixability as that of a steel sheet, unless the yield strength of the aluminum alloy sheet in press working is considerably decreased. However, when it is intended to obtain a structure having a tensile strength of about 300 MPa comparable to that of a steel sheet, the yield strength of the aluminum alloy sheet manufactured by the conventional method is inevitably increased to about 140 MPa or above in both of the 5000 series alloy and the 6000 series alloy, which is likely to give rise to a poor shape fixability.
Thus, excellent formability, shape fixability, high strength, dentability and corrosion resistance are required of the sheet material used as body panels of automobiles. However, the shape fixability, high strength and dentability are properties contrary to each other. Accordingly, the development of the sheet material which can meet all the requirements has been desired in the art.
On the other hand, a proposal has been made on a molding Al alloy sheet having excellent weldability, filiform corrosion resistance, formability and bake hardenability manufactured by subjecting and Al-1%Mg-1%Si-based aluminum alloy sheet material to solution heat treatment through rapid heating and rapidly cooling the treated material to regulate the grain size and the electrical conductivity to respective particular values (see Japanese Patent Application Laid-Open No. 64-65243). Further, the present inventors have proposed a process for manufacturing an aluminum alloy for forming having excellent shape fixability and bake hardenability, which comprises subjecting an Al-Si-Mg-based aluminum alloy sheet material to solution heat treatment through rapid heating, rapidly cooling the treated material, allowing the cooled material to stand at room temperature for a period of time as short as possible and heating and holding the material at a temperature of from 50 to 150.degree. C. (see Japanese Patent Application Laid-Open No. 2-269508).
As described above, in 5000 series aluminum alloys, although the formability is excellent, when a tensile strength of 300 MPa or more comparable to that of a steel sheet is intended, the yield strength becomes 140 MPa or more, so that no shape fixability can be attained in press working. On the other hand, in 6000 series aluminum alloys, the paint baking temperature is so low that no sufficient strength can be attained. Further, the formability lowers due to room temperature age hardening, and the corrosion resistance is poor.
In order to eliminate the above-described problems, Japanese Patent Application Laid-Open No. 64-65243 and U.S. Pat. No. 4,909,861 (Muraoka et al.) propose a process for manufacturing a material having an excellent bake hardenability. In this process, a heat treatment is further conducted within 72 hours after the solution heat treatment and cooling. However, reheating is necessary, and the bake hardenability in working examples is unsatisfactory for actually reducing the weight. In order to reduce the weight by 10% as compared with the conventional 5000 series alloys, a bake hardenability of about 50 MPa appears to be necessary although it depends upon the shape of the body.
Patent applications relevant to Japanese Patent Application Laid-Open No. 64-65243 have been filed by the same assignee (see Japanese Patent Application Laid-Open Nos. 62-89852, 62-177143, 1-111851, 2-205660 and 3-294456). Among them, Japanese Patent Application Laid-Open No. 1-111851 discloses that when the hardening is conducted by allowing the material to stand at room temperature below 60.degree. C., the bake hardenability at a temperature as low as about 170.degree. C. disappears with prolonging of the hardening time. Further, Japanese Patent Application Laid-Open No. 2-205660 discloses that the properties lower once the temperature is lowered to room temperature, and in the working example of this Patent Application, there is a description to the effect that the bake hardenability lowers when the material is allowed to stand for a long period of time. For this reason, in order to attain sufficient hardening, as described above, it is preferred to conduct a heat treatment within a time as short as possible, that is, one hour, after cooling.
In the manufacture of a body sheet material on a commercial scale, however, since a continuous annealing furnace is used in the solution heat treatment and cooling, the material is treated in the coil form. For this reason, it is difficult to transfer the material to the next step within one hour to conduct a heat treatment, so that there occurs a problem in an actual operation.
Japanese Patent Application Laid-Open No. 1-111851 discloses that the material after the solution heat treatment is cooled to 60 to 130.degree. C. and held at that temperature. In the treatment of the material in the coil form on a commercial scale, it is very inefficient and difficult to hold the material at the above-described temperature for a long period of time (0.5 hour or longer).
The provision of the limitation of the time for transfer to the next step is unfavorable from the viewpoint of production on a commercial scale even when the time requirement is such that the material is transferred to the next step after the solution heat treatment and cooling without any additional treatment, or within 72 hours after hardening. The process which comprises conducting a similar solution heat treatment, allowing the treated material to stand at room temperature for a period of time as short as possible and heating and holding the material at 50 to 150.degree. C. has a drawback that the step of reheating becomes necessary after the solution heat treatment.